1. Field of the Invention
The present invention is directed to the method and apparatus for forming conductors directly on a strander in the manufacture of compacted strand.
2. Description of the Prior Art
Electric cables customarily are made by stranding together a plurality of conductors in layers, a first layer of which surrounds a core comprising a single center conductor, while a second layer surrounds the first layer, and so on. The geometry of the cable cross-section is such that six conductors fit firmly around the single center conductor, twelve conductors fit firmly around the six, eighteen conductors fit firmly around the twelve, and so forth, with the number of conductors forming each cable equal exactly to 3n.sup.2 +3n+1 wherein n represents the number of layers surrounding the single center conductor.
Concentric layer conductors have large overall diameters for a given conductance, or cross-section of metal. This is because of the plurality of open spaces or interstices found between each of the conductors. It is known in the prior art that the conductors of concentric lay conductors may be crushed or compacted after the layers have been applied thereby to cause the metal of a conductor to flow into a space or interstice and in this manner to reduce the overall diameter of the conductor. A patent of the prior art illustrating an apparatus and method for accomplishing this end is U.S. Pat. No. 1,943,087 to F. M. Potter et al. Potter et al, which is still rather representative of the state of the art, utilizes a plurality of compacting rolls comprising rolling units in the manufacture of round compacted strand. In Potter et al, round wire is stranded and compacted at a roll station and according to Potter et al each succeeding layer is stranded and compacted in the same lay direction. The compacting of a standard conductor, however, changes the geometrical dimensional relationships so that a layer with six more wires than the underlying layer no longer sits naturally onto the conductor. To compensate for the reduction in diameter of the conductor core by compacting, the individual diameters of wires in succeeding layers are disclosed as being reduced in dimension. While this has a significant advantage in that all of the carriers of the stranding machine are utilized, it also introduces a serious disadvantage in that a multiplicity of different wire sizes are required to be drawn and stocked. A further disadvantage of the Potter et al patent relates to the necessity to reduce the length of stranding lay substantially below the maximum allowed by the American Society for Testing Materials (ASTM) and other industry standards. This is 16 times the diameter for copper (see ASTM B 496-69) and for Class B strand aluminum (see ASTM B 400- 70). Shortening of the lay adds significantly to the cost of manufacturing since it reduces the hourly production of the stranding machines.
The aforementioned patent teaches that conductors may be compacted by means of pressure rolls. More recently, it has also been suggested to carry out the compaction of conductors by pulling the conductors through a wire drawing-type die after application of each layer of conductors. However, conductors fabricated by either of the methods or apparatus, using industry teachings for the selection of wire size and lay lengths, has been characterized by wide varieties of temper, due to uneven work hardening through the section of the conductor. Typically, the central wires have been excessively hardened, and it has been found that failure occurs within the central wires when the conductors are flexed or tensioned at an earlier period of time than would be expected for a conductor of uniform hardness throughout its section and, as a result, the load is not evenly distributed and failure will occur earlier than in a conductor of uniform hardness throughout.
A very early patent wherein there is a forming operation although not on a strander is U.S. Pat. No. 433,917 to E. P. Warner. The Warner patent, thus, relates to a method of manufacturing electric conductors and describes that a pair of strands coated with a winding or braiding may be passed through a pair of revolving dies to effect a change in the cross-section in each strand. The strand is round and assumes a D-shaped configuration upon operation of the dies. The two strands are laid together along their flat faces, caused to move to a die and bound together with a serving. The Warner patent, while it does disclose the forming of a pair of strands from a round to a D-shaped configuration, neither discloses nor does it suggest that the individual strands are formed on a strander, and the strands are not compacted thereby to be reduced in cross-section. The strands are only subject to a rearrangement of their geometrical cross-section.